I’ve just heard on CNN that Serena Williams pulled out of the Sony Ericsson Championships in Madrid.

Unbelievable, both the Williams sisters had qualified for the season-ending event, but had to withdraw. Venus said some time before the tournament that she wouldn’t compete because of dizziness she had suffered during the US Open.

World No.5 Serena Williams retired from her match against Russian Anna Chakvetadze on Wednesday, due to left knee injury, but was hoping to continue her fight for the Sony Ericsson Championships title. What a disappointment!

The American will be replaced by France’s Marion Bartoli. (photo via Getty Images)

Serena Williams retired with a left knee injury while trailing 6-4 to Anna Chakvetadze at the Sony Ericsson Championships. The 26-year-old American, who won the event in 2001, hopes that she will be able to continue to compete in Madrid.

"Before the match I had taping for prevention since I was feeling a little tight in both knees. I thought I was tighter in the right knee but then, once I had to increase my range of movement, the left knee got worse," said world No.5 Williams.

"I've had issues with my knees before especially the left knee since that's the one I had surgery on in 2003. It's unfortunate it's happened now and I can't stress how disappointed I am.

"Right now it's swollen and I'm going off to have an MRI at the hospital to see exactly what the problem is. It is my intention to play on in this event."

I’m still waiting for the news whether Serena will continue to play in Madrid. If not, France’s Marion Bartoli will step in. (source: Reuters, photo via Getty Images)

Serena Williams was forced to retire from her first match at the 2007 Sony Ericsson Championships because of a left knee injury. Having lost the opening set 4-6, Williams shook hands with Chakvetadze, and left the court in tears.

The Russian was sad that she won the match in this way, but the triumph is very important to her since it keeps her in competition for the semifinals (Chakvetadze lost her first match at the WTA Championships to defending champion Justine Henin).

I still don't know if Serena will be able to quickly recover from the injury and continue to compete at the season-ending Championships, but if not, Marion Bartoli will take her place as she is the first alternate. (photo via Getty Images)

World number six Serena Williams is the fifth player to qualify for the season-ending Championships, after Justine Henin, Jelena Jankovic, Svetlana Kuznetsova and Ana Ivanovic. By reaching the final of the Kremlin Cup in Moscow, Williams has achieved this feat for the seventh time in her career.

The Sony Ericsson Championships will take place in Madrid form November 6-11. It is the richest annual women's sporting event in the world which gathers the Top 8 singles players and Top 4 doubles teams from the season to battle it out for a share of the $3-million prize purse. And don't forget that from 2008 prize money will be even greater. (source: WTA Tour)

Elena Dementieva claimed her first Kremlin Cup title on Sunday with a 5-7 6-1 6-1 victory over fourth-seeded Serena Williams in their fifth career meeting.

This title has a special significance to the Russian, since it is her first title on home soil in her 11th Kremlin Cup appearance. Before she had lost two finals at the tournament in Moscow, in 2001 and 2004.

After losing the first set, world No.14 Dementieva came back strong, breaking Williams three times in each of the next two sets. During the match, Williams made 59 unforced errors to her opponent’s 31.

"She just played really unbelievable, she should play like that more often," said world No.7 Serena Williams.

"I'm not going to make excuses for myself. I played terrible. I had a feeling that almost every shot I hit, it was out."

Having won her eighth career title, Dementieva earned $182,000 and took her record against Williams to 1-4. (source: Reuters, photo via Yahoo)